1. Field of the Invention
The present invention relates to a method of detecting the detachment of the venous needle from a patient during an extracorporeal blood treatment in a dialysis machine.
2. Description of the Related Art
As is known, blood consists of a liquid component called the blood plasma and a corpuscular component formed by the blood cells, including the red corpuscles among other components. In renal insufficiency, the blood has, in addition to the aforesaid components, particles of low molecular weight (referred to below as solute) which have to be eliminated by a dialysis treatment carried out with a dialysis machine.
A dialysis machine of the known type generally comprises an extracorporeal blood circuit, a dialysate circuit and a filter, which is located in the aforesaid circuits and comprises a blood compartment and a dialysate compartment, which are separated from each other by a semi-permeable membrane, and through which pass, respectively, the blood to be treated and the dialysate, generally flowing in counter-current mode.
During the dialysis treatment, the undesired particles contained in the blood migrate from the blood compartment to the dialysate compartment through the semi-permeable membrane both by diffusion and by convection, as a result of the passage of some of the liquid contained in the blood towards the dialysate compartment. Thus the patient will have lost some weight by the end of the dialysis process.
The extracorporeal circuit is connected to the patient by means of an arterial needle and a venous needle, which are inserted into fistulas formed in the patient's cardiovascular system, so that they can, respectively, collect the blood to be treated and return the treated blood to the patient's cardiovascular system. The extracorporeal circuit comprises a peristaltic pump and a dropper located in the arterial branch and in the venous branch respectively. The detachment of one of the aforesaid needles from the fistula causes an interruption of the access to the patient's cardiovascular system. The detachment of the venous needle, if not detected in good time, has particularly serious consequences, because it can cause a significant blood loss in the patient. Various attempts have therefore been made to provide methods for detecting the detachment of the aforesaid needles, and particularly of the venous needle.
One of the aforesaid known methods is described in WO 99/12588. This method is based on the electrical conductivity of the blood, and consists of the injection of a current into a closed circuit consisting of the extracorporeal circuit and the patient's cardiovascular system, and the measurement, by means of a measuring instrument located in the aforesaid extracorporeal circuit, the current variations which are caused by the detachment of one or both of the needles. For this method, the current injection and the measurement of the current variation have to be carried out by inductive coupling, in other words by means of windings located at specified points along the extracorporeal blood circuit.
The method described above has various drawbacks. In particular, this method, although theoretically valid, cannot provide satisfactory results from the practical point of view, since the high electrical impedance produced by the peristaltic pump, which effectively interrupts the continuity of the blood flow, makes it necessary to operate with relatively high currents in order to make use of the low conductivity of the materials, generally PVC, which are used to form the extracorporeal circuit, the filter, the peristaltic pump and the dropper. The use of relatively high currents is most inadvisable in a machine connected to a patient, and, even if these currents could be used, it would not be possible to transmit them by means of an inductive coupling, which, among other considerations, generates additional parasitic currents which interfere with measurements. In some dialysis machines, the dropper also creates a high impedance, of the same order of magnitude as that created by the peristaltic pump, and thus exacerbates one of the problems described above.
Consequently, because there is a requirement to operate with relatively low currents, and because the impedance of the peristaltic pump, and of the dropper in most cases, is high, the detachment of one of the needles causes current variations which are not easily identifiable and which can be confused with the background noise of the measuring instrument.
Furthermore, this method does not allow for the fact that the patient may be connected to earth and that the filter itself is necessarily connected to earth, since the dialysate circuit is connected to earth to meet the requirements of the safety regulations for dialysis machines. Therefore the electrical circuit assumed in the aforesaid application does not truly represent the real analogy of a dialysis machine in electrical terms.
It is also known from U.S. Pat. No. 6,221,040 a method for monitoring vascular access using pressure signals in a way different from the present invention.
The object of the present invention is to provide a method of detecting the detachment of the venous needle from a patient during an extracorporeal blood treatment in a dialysis machine, which overcomes the drawbacks of the known art.